A new approach to electrical heating appliances in recent years has been self-regulating heating systems which utilize materials exhibiting certain types of PTC (positive temperature coefficient) of resistance characteristics. The distinguishing characteristic of the prior art PTC materials is than upon attaining a certain temperature, a substantial rise in resistance occurs. Prior art heaters utilizing PTC materials generally exhibit more or less sharp rises in resistance within a narrow temperature range, but below that temperature range exhibit only relatively small changes in resistance with temperature. The temperature at which the resistance commences to increase sharply is often designated the switching or anomaly temperature (T.sub.S) since on reaching that temperature the heater exhibits an anomalous change in resistance and tends to switch off. Unfortunately, such switch-off only occurs at relatively low power densities with prior art PTC elements.
Self-regulating heaters utilizing PTC materials have advantages over conventional heating apparatus in that they generally eliminate the need for thermostats, fuses or in-line electrical resistors.
The most widely used PTC material has been doped barium titanate which has been utilized for self-regulating ceramic heaters employed in such applications as food warming trays and other small portable heating appliances. Although such ceramic PTC materials are in common use for heating applications, their rigidity has severely limited the number of applications for which they can be used. PTC materials comprising electrically conductive polymeric compositions are also known and certain types have been shown to possess the special characteristics described herein-above. However, in the past, use of such polymeric PTC materials have been relatively limited, primarily due to their low heating capacity. Such materials generally comprise one or more conductive fillers such as carbon black or powdered metal dispersed in a crystalline thermoplastic polymer. PTC compositions prepared from highly crystalline polymers generally exhibit a steep rise in resistance commencing a few degrees below their crystalline melting point similar to the behavior of their ceramic counterparts at the Curie temperature (the T.sub.S for ceramics). PTC compositions derived from polymers and copolymers of lower crystallinity, for example, a crystallinity or less than about 50%, exhibit a somewhat less steep increase in resistance which commences at a less well defined temperature range often considerably below the crystalline melting point. In the extreme case some polymers of low crystallinity yield resistance temperature curves which are more or less concave upwards with no defineable inflection point. Other types of thermoplastic polymers yield resistance temperature curves which increase fairly smoothly and more or less steeply but continuously with temperature. FIG. I illustrates characteristic curves for the aforementioned different types of PTC compositions. In FIG. I curve I exhibits the sharp increase in temperature (herein-after known as type I behavior) characteristic of (inter alia) barium titanate and polymers having very high crystallinity, curve II shows the more gradual increase at lower temperatures (relative to the polymer melting point) hereinafter known as type II behavior characteristic of most medium to high crystallinity polymers. Curve III (Type III behavior) exhibits the curve concave upward characteristic of many very low crystallinity polymers while curve IV (Type IV behavior) illustrates the large increase in resistance without any region of more or less constant resistance (at least in the range of commercial interest) seen with some materials. Curve V (Type V behavior) illustrates the gently increasing resistance temperature characteristic shown by many prior art electrical resistors. Although the above types of behavior have been illustrated by reference to specific types of material it will be realized by those skilled in the art that the type of behavior is also very significantly influenced by the type and amount of conductive filler present and, particularly in the case of carbon black filler, its particle size, surface characteristics, tendency to agglomerate and the shape of the particles or particle agglomerates (i.e. its tendency to structure).
It should be noted that although the prior art references teach only compositions purportedly manifesting Type I behavior, we have found that such prior art compositions in fact usually manifest Type II to Type IV behavior, which alternative Types are unrecognized by the prior art. Additionally, even those prior art materials which do have a distinct anomaly point, i.e., undergo a sharp increase in resistance at T.sub.s encounter a fall-off, i.e., decrease in resistance if the temperature of the PTC element increases significantly above T.sub.s which can occur, particularly when high power densities are present in the element.
Kohler, U.S. Pat. No. 3,243,753 discloses carbon filled polyethylene wherein the conductive carbon particles are in substantial contact with one another. Kohler contemplates a product containing 40% polyethylene and 60% carbon particles so as to give a resistance at room temperature of about 1 ohm/in. As is typical of the alleged performance of the prior art materials, Kohler's PTC product is purportedly characterized by a relatively flat curve of electrical resistance versus temperature below the switching temperature followed by a sharp rise in resistivity of at least 250% over a 25.degree. F. range (i.e., Type I behavior). The mechanism suggested by Kohler for the sharp rise in resistivity is that such change is a function of the difference in thermal expansion of the materials, i.e. polyethylene and particulate carbon. It is suggested that the composition's high level (i.e. 60%) of conductive filler forms a conductive network through the polyethylene polymer matrix, thereby giving an initial constant resistivity at lower temperatures. However, at about its crystalline melt point, the polyethylene matrix rapidly expands, such expansion causing a breakup of many of the conductive networks, which in turn results in a sharp increase in the resistance of the composition.
Other theories proposed to account for the PTC phenomenon in conductive particle filled polymer compositions include complex mechanisms based upon electron tunnelling through inter grain gaps between particles of conductive filler or some mechanism based upon a phase change from crystalline to amorphous regions in the polymer matrix. A background discussion of a number of proposed alternative mechanisms for the PTC phenomenon is found in "Glass Transition Temperatures as a Guide to the Selection of Polymers Suitable for PTC Materials", J. Meyer, Polymer Engineering and Science, November 1973, Vol. 13, No. 6. Of significance is the fact that the PTC polymeric materials of the prior art contemplate compositions which exhibit a T.sub.S at or below the melting point of a thermoplastic component.
As mentioned above, Kohler, discloses a polyethylene or polypropylene-carbon black polymeric matrix, in which the polyolefin has been polymerized in situ, such materials exhibiting PTC characteristics at the melting temperature of the polymers. Likewise, Kohler discloses carbon particles dispersed in polyethylene in which the composition may be crosslinked, or may contain a thermosetting resin to add strength or rigidity to the system. However, the T.sub.S temperature still remains at about the crystalline melting point of the thermoplastic polyethylene, i.e., 120.degree. C.
U.S. Pat. No. 3,825,217 to Kampe discloses a wide range of crystalline polymers which exhibit PTC characteristics. These include polyolefins such as low, medium, and high density polyethylenes and polypropylene, polybutene-1, poly(dodecamethylene pyromellitimide), ethylenepropylene copolymers, etc. It is also suggested that blends or crystalline polymers such as, a polyethylene with an ethylene-ethyl acrylate copolymer may be employed for the purpose of varying the physical properties of the final product. Also disclosed by Kampe is a process of thermal cycling above and below the melting temperature of the polymers to achieve a lower level of resistance. Similarly, Kawashima et al, U.S. Pat. No. 3,591,526 discloses polymer blends containing carbon black exhibiting PTC characteristics. However, again the thermoplastic material dictates the T.sub.S temperature, such temperature occurring at about its crystalline melting point, while the second material is functioning merely as a carrier for the carbon black loaded thermoplastic.
Finally, commonly assigned U.S. Pat. No. 3,793,716 to Smith-Johannsen discloses conductive particle-polymer blends exhibiting PTC characteristics in which a crystalline polymer having dispersed therein carbon black is dissolved in a suitable solvent above the polymer melting point which solvent is then evaporated to afford a composition manifesting a decrease in room temperature resistivity for a given level of conductive filler. Again the T.sub.S temperature is at or below the melting point of the polymer matrix, and the process of heating the polymer above the melting temperature is directed at decreasing resistance and/or maintaining constant resistance at ambient temperatures.
Current self-regulating thermal devices utilizing a PTC material contemplate, as above indicated, but do not in fact provide extremely steep (Type I) R=f (T) curves so that above a certain temperature the device will effectively shut off, while below that temperature a relatively constant wattage output at constant voltage is achieved. At temperatures below T.sub.S the resistance is at a relatively low and constant level and thus the current flow is relatively high for any given applied voltage (I=E/R). The power generated by this current flow is disspated as Joule heat, i.e. heat generated by electrical resistance=I.sup.2 R, thereby warming up the PTC material. The resistance stays at this relatively low level until about the T.sub.S temperature, at which point a rapid increase in resistance occurs. With the increase in resistance there is a concomitant decrease in power, thereby limiting the amount of heat generated so that when the T.sub.S temperature is reached heating is essentially stopped. Then, upon a lowering of the temperature of the device below the T.sub.S temperature by dissipation of heat to the surroundings, the resistance drops thereby increasing the power output. At a steady state, the heat generated will balance the heat dissipated. Thus, when an applied voltage is directed across a Type I PTC heating element, the Joule heat causes heating of the PTC element up to about its T.sub.S (the rapidity of such heating depending on the type of PTC element), after which little additional temperature rise will occur due to the increase in reistance. Because of the resistance rise, such a PTC heating element will ordinarily reach a steady state at approximately T.sub.S thereby self-regulating the heat output of the element without resort to fuses or thermostats. The advantages of such a self contained heat regulating element in many applications should be apparent, in that the need for expensive and/or bulky heat control devices such as thermostats is eliminated.
Obviously, from the preceeding discussion, those skilled in the art consider materials manifesting Type I behavior to have significant advantages over PTC materials showing other types of behavior. Types II and III have a major disadvantage in that because of the much less sharp transition the steady state temperature of the heater is very dependent on the thermal load placed on it. Such materials also suffer from a current inrush problem as described in greater detail hereinafter. Type IV PTC materials, because they lack a temperature range in which the power output is not markedly dependent on temperature have so far not been considered as suitable materials for practical heaters.
Although, as hereinabove mentioned, the prior art recognizes the considerable advantage of having a heater composition which possesses a resistance-temperture characteristic of Type I, many of the allegedly Type I compositions alluded to in the prior art in fact show behavior more closely resembling Type II, or even Type III behavior. The optimum (Type I) behavior is shown by only a limited selection of compositions and there has been a long felt need for a means of modifying compositions showing Type II or III behavior which on the basis of physical or other characteristics would be useful for PTC heating elements so that their behavior more closely approaches Type I. Furthermore, as heretofore indicated, many prior art materials although showing a more or less sharp inflection at T.sub.s can be caused to "turn on" again at termperatures slightly above T.sub.s. That is, if the increase in resistance above T.sub.s is not great enough and/or if resistance drops above the compositions melting point (as is generally the case with prior art materials) then thermal runaway and burn-out can occur.
Polymeric PTC compositions have also been suggested for heat shrinkable articles. For example, Day in U.S. Patent Office Defensive Publication No. T905,001 teaches the use of a PTC heat shrinkable plastic film. However, the Day shrinkable film suffers from the rather serious shortcoming that since T.sub.s is below the crystalline melting point of the film, very little recovery force can be generated. Neither Day nor any of the other previously discussed prior art teachings even address themselves to, much less solve, certain additional problems inherent in all prior art PTC heaters. First, is the problem of current inrush. This problem is particularly severe when it is desired to provide a heater having a T.sub.S in excess of about 100.degree. C. While it is feasible to find a polymeric PTC material having a T.sub.S as high as 150.degree. C., the resistance of such material at or just below the T.sub.S may be as much as 10 times its resistance at ambient temperature. Since the PTC heater ordinarily functions at or slightly below its T.sub.S, its effective heat output is determined by its resistance at slightly below T.sub.S. Therefore, a PTC heater drawing, for example 50 amps at 150.degree. C. could easily draw 500 amps at ambient temperatures.
When one desires to use a heat recoverable material comprising a PTC heater further deficiencies of compositions exhibiting current inrush appear. It is advantageous for heat recoverable articles to shrink as rapidly as possible. Obviously a heater having a flat power/temperature characteristic will heat up more rapidly and uniformly than a heater having, for example, a power output which is one tenth of its ambient temperature value at just below its T.sub.S. We have found that selection of polymers of high crystallinity as the matrix for the conductive particles minimizes this aforesaid current inrush problem. Furthermore, such high crystallinity polymers exhibit a steep increase in resistance (i.e. have a T.sub.S) about 15.degree. C. below their crystalline melting point. Unfortunately, such polymers still possess considerable crystallinity at T.sub.S and thus not only show little recovery if previously converted into a heat recoverable state, but resist recovery of associated heat recoverable members which may themselves be above their recovery temperature. Obviously, if one selects heater resistances (i.e. lower resistances) such that the heater is switched off at a temperature closer to its peak resistance temperature (T.sub.p), which correspond closely to the actual melting point, the aforementioned disadvantage may be avoided. However, we have found that all prior art heaters show resistances which either decrease sharply or in a very few instances stay substantially constant as the temperature of the PTC material is increased above its melting point. Another shortcoming of prior art PTC polymeric compositions is that as they are elongated (say to form a heat recoverable object) the ratio of the resistance at T.sub.p to the resistance at T.sub.s decreases dramatically. Thus an initial ratio of 10.sup.8 may fall to 10.sup.5 at 10% elongation and 10.sup.3 at 25% elongation. Obviously, these last factors greatly increase the potential for runaway overheating with prior art heaters when used in heat shrinkable devices.
It would therefore substantially advance the art to provide a PTC material evidencing Type I behavior, and which does not suffer from severe current inrush. Surprisingly, we have found that many of the hereinabove discussed deficiencies of the prior art may be remedied by the provision of a polymeric, thermoplastic electrically conductive composition which exhibits a sharp rise in resistance just below its melting point but whose resistance continues to rise as the temperature is increased above the melting point. Heaters having this characteristic will continue to control even if their temperature rises above the melting point of the thermoplastic polymer, while prior art heaters would suffer thermal runaway and perhaps burn out under these conditions.
By provision of PTC compositions having this characteristic we are able to obtain heaters which will control at a resistance level even considerably above the resistance level at T.sub.s without substantial risk of thermal runaway and burn out. Furthermore, because the resistance continues to increase above T.sub.s and above the melting point, the heater temperature under power shows very little change under conditions which vary from low to high thermal loads. Thus heaters made from the compositions of the present invention reach their operating range in about the same period of time irrespective of the thermal environment within wide limits and are very "demand insensitive". This insensitivity of the heater temperature to the thermal load enables the manufacturer of heat shrinkable devices to design products which have a high degree of art insensitivity and which will not damage the substrate, such as a thermoplastic cable jacket, onto which the device is recovered.
Thus there exists a long felt need for a heater composition which overcomes the hereinabove enumerated defiencienes of the prior art.
It is thus an object of this invention to provide polymer compositions suitable for PTC heaters whose resistance changes very little below the T.sub.s.
It is a further object of this invention to provide PTC polymer compositions having a T.sub.s above 100.degree. C.
It is a further object of this invention to provide PTC polymer compositions which manifest a continued increase in resistance above the melting point of said composition.
It is another object of this invention to provide polymer compositions suitable for use as heaters in heat recoverable devices.